Continuously operating process valves are used for flow control in industrial processes, in particular in the chemical and petrochemical industry, and in power stations. Such process valves are mainly actuated via pneumatically operated actuators. Such actuators basically comprise a cylinder in which a piston is moved by the pneumatic fluid. This piston is coupled mechanically to the process valve. The pneumatic fluid exerts pressure on the cylinder by means of a positioner, as it is called.
Two types of actuators are known by prior public use. In the “single-acting” actuator, the pneumatic fluid exerts pressure on just one side of the piston by means of the positioner. A spring is provided to move the piston in the opposite direction. In the double-acting actuator, the pneumatic fluid exerts a pressure on both sides of the piston by means of the positioner. This means that it is possible to move the piston, and hence the process valve coupled to the piston, in both directions without mechanical return elements.
The positioner is provided with an electrical setpoint value for the valve position of the process valve via a communications interface. This communications interface may have a digital implementation as a fieldbus interface or an analog implementation as a 4.20 mA interface.
The actual value of the valve position is detected via a sensor element on the mechanical coupling between the actuator and the process valve. The positioner then controls at its output the pressure of the pneumatic fluid such that the valve position attains or retains the defined setpoint value.
Although in such a positioner the defined setpoint value of the valve position is attained quickly by controlling the valve position, in the automation engineering application, the actual position of the process valve is only of secondary importance. In contrast, process parameters such as the volume flow rate in a pipeline or the level in a vessel are far more relevant to the automation engineering application. In controlling the valve position, a higher-level controller than the positioner, located in the control system, must convert these process parameters into the setpoint value defined for the process valve, namely its valve position, and transfer it to the positioner. The crucial process parameter is adjusted with a corresponding time delay. In addition, it is deemed disadvantageous that a control system is needed, which defines the setpoint values for the positioner. Since the positioner has no information available on the real process parameter, monitoring functions can only be implemented by separate software modules in the control system.
In addition, fitting and calibrating the valve-position detection system is very time consuming. It requires a reference movement of the valve and hence intervention in the process.